The present invention concerns fine-particle-size ion exchange resins and methods for their preparation. In particular it concerns spherical, crosslinked emulsion copolymer particles in a size range of from about 0.1 to about 1.5 micrometers in diameter, which bear ion exchange functional groups, and emulsions of these particles. It further concerns the preparation of these particles and emulsions, and the use of these particles and emulsions in removing dissolved and undissolved material from liquids.
Finely divided ion exchange materials have been used extensively as filter media for the simultaneous filtration and deionization of condensate water from steam turbine generators, and to a lesser extent in pharmaceutical applications such as drug carriers and tablet disintegrators, and in other commercial applications.
In the past such finely divided ion exchange materials have been produced by grinding or otherwise physically reducing the size of ion exchange particles produced by conventional processes involving the separate steps of polymerization--most commonly suspension polymerization--and functionalization.
Schultz and Crook (I & EC Product Research and Development, Vol. 7, pp. 120-125, June, 1968) have produced particles of ground ion exchange resin with average diameters of one micron or smaller, but the particles are not spherical, and the range of diameters within a given sample of such materials is large, i.e., the particles are not uniformly sized. Even though large particles may constitute only a small fraction of the total number of ground particles, they represent a much larger fraction of the sample weight. As a result, such ground resins exhibit settling of a significant fraction of the ion exchange material weight from aqueous suspension.
Suspension polymerization involves suspending droplets of organic liquid containing monomers, polymerization initiators and suspension stabilizers in an aqueous-phase medium. The droplet size, largely a function of agitation rate, controls the final polymer particle size, which normally ranges down to about 40 micrometers, although sizes down to 5 micrometers (U.S. Pat. No. 3,357,158) or 10 micrometers (U.S. Pat. No. 3,991,017) have been disclosed. Ion exchange materials have also been produced by bulk polymerization. Physically reducing the particle size of such polymers in bulk or bead form to sub-micron sizes is difficult and expensive, and produces material with undesirable physical characteristics such as irregular particle shape and broad particle-size distribution. It may also produce undesirable heat degradation of the resin.
Sub-micron sized, spherical polymer particles have been prepared in the past, including some with limited ion exchange functionality. These particles were prepared from monomers which contained ion exchange functional groups, such as acrylic and methacrylic acid, or dialkylaminoalkyl acrylates and methacrylates. In most cases the polymerization reaction used was emulsion polymerization. Thus Haag et al (U.S. Pat. No. 3,847,857) used ". . . from 5 to 70% by weight . . . of one or more monomers containing an amine or quaternary ammonium group . . ." (column 2, lines 56-59) in forming a functional, crosslinked emulsion ion exchange resin for use in paints and other coatings. Rembaum et al (U.S. Pat. No. 3,985,632) similarly prepared chromatographic absorbents by emulsion polymerizing monomer mixtures containing minor amounts of monomers with amine functionality (column 5, lines 25-46). Fitch (U.S. Pat. No. 3,104,231) used up to 15% by weight of monomers containing carboxylic acid groups when preparing crosslinked emulsion copolymers. He cautions that higher content of such monomers leads "to either solubility of the copolymer in water or dilute alkali or significant swelling of the copolymer in such aqueous media." (column 6, line 70--column 7, line 4).
Hatch (U.S. Pat. No. 3,957,698) describes a precipitation polymerization for making crosslinked, spherical ion exchange resin particles in a size range similar to that of emulsion polymer particles. The precipitation process inherently produces larger particles, in the range of 0.1-10 micrometers (compare 0.01-1.5 mm for emulsion polymerization), and involves the precipitation of polymer particles from a monomer-solvent solution in which polymer is insoluble. In emulsion polymerization the monomer is only slightly soluble in the solvent, and the polymer particles are formed when monomer-swollen soap mycelles contact solvent-phase-initiated polymer chains. Hatch mentions that "suitable micro bead resins can be prepared by suspension or emulsion polymerization . . ." He then describes suspension polymerization but fails to indicate any detail of an emulsion polymerization process (column 3, lines 30-40). The ion exchange microbeads of Hatch are weak acid resins made from carboxylic acid monomers such as acrylic or methacrylic acid, although the use of esters of these acids is mentioned, with hydrolysis subsequent to polymerization. Hatch exemplifies the preparation of a microbead from vinylbenzyl chloride (Example 4), but the particle size (3-7 microns) is clearly outside the range of the present invention, and no attempt is made to impart ion exchange functionality to the microbead itself until it has been incorporated in an ion exchange resin matrix. Hayward (U.S. Pat. No. 3,976,629) also prepared weakly acidic cation exchange resins of a size "less than 20 microns" using a modified suspension polymerization and carboxylic acid monomers.
Tamura (Nippon Kagaku Kaishi 76 (4), pages 654-8, 1976) discloses the preparation of strongly acidic cation exchange resin material from emulsion copolymers. Tamura coagulated styrene-divinylbenzene copolymer emulsions and functionalized them with fuming sulfuric or chlorosulfuric acids. He subsequently mixed the coagulum into a polypropylene membrane, but did not teach that the coagulum might be re-emulsified.